I. Field of the Invention
This invention relates to directional microwave antenna systems and more particularly to a radar antenna system for use with an airborne radar. The antenna system comprises a parabolic dish reflector fed by a wave guide having two apertures and a splash plate positioned at the focus of the reflector. The antenna system produces a pencil beam which can be steered in azimuth and elevation by displacement of the parabolic reflector.
II. Description of the Prior Art
Directional microwave antenna systems such as that disclosed in U.S. Pat. No. 2,422,184 provide a pencil beam antenna pattern particularly adapted for use in airborne radar systems. These prior art systems comprise a parabolic reflector fed by a wave guide having apertures and a splash plate positioned at the focus of the reflector. The beams of these prior art antenna systems can be steered by movement of either the feed or the reflector or by pointing the entire antenna assembly. These systems are particularly useful in airborne systems where space is at a premium and where it is desirable to mount the radar antenna behind a radome.
Antenna systems used with airborne radars usually employ a relatively narrow beam to achieve high resolution. Because the magnetron transmitting tubes used with such radar systems are sensitive to reflected energy, the radar antenna system should reflect little energy, i.e. the voltage standing wave ratio (VSWR) of the antenna should be low. The side and back lobes of the radar antenna pattern must be maintained as low as possible, at least 20 db down, to avoid false target ambiguities.
The directional microwave antenna system disclosed in U.S. Pat. No. 2,422,184 to Cutler has, with some variations, been widely used in airborne radar antenna systems. This standard Cutler Cutler has a relatively high VSWR and thus reflects considerable energy back into the magnetron of the radar system. This reflected energy poses substantial design constraints because the radar system's automatic frequency control circuitry must be able to maintain lock with the desired signal in the presence of random-phase interfering reflected signals.
To overcome the reflected energy difficulties associated with the standard Cutler feed, it has been common practice to tune the antenna system by inserting tuning stubs in the wave guide of the antenna. The positioning of these tuning stubs is frequency sensitive and thus while the tuned Cutler feed system represents an improvement of the standard Cutler feed at a specific frequency, the system is very narrow-band. Further, there is a variation in the overall reflected power of the system as the parabolic reflector is moved from one position to another. This variation is exhibited by all Cutler feed antennas but is particularly pronounced for the stub-tuned Cutler feed.